"From Surface Science to Device Engineering in Organic Optoelectronics"

Chih-I Wu, Associate Professor, Graduate Institute of Photonics and Optoelectronics, National Taiwan Univeristy

August 29th (Monday), 3:00pm
Materials Research Lab (MRL), Rm 2053

One of the important factors determining the performance of organic optoelectronic devices is the current efficiency of devices. We will present a systematic study on the relations between the device performance and interactions at the metal-organic and organic-organic interfaces. The effects of gap states and energy level alignments on the carrier injection mechanisms in the organic light emitting devices are investigated. The performances of devices with various combinations of anode and cathode structures are compared to find out effects of interfacial band alignments on turn-on voltages of OLEDs. To find out the key factors affecting carrier injection efficiency, the interfacial electronic structures and chemical properties were then studied with using X-ray and ultra violet photoemission spectroscopy (XPS and UPS). XPS and UPS data indicate that although energy alignments are still the keys to the efficient carrier injections, the gap states, in these cases, play more important roles to help the carriers move over the barriers and inject into the organic materials. The origins of the gap states will also be discussed.

On the other hand, while the current density versus voltage (J-V) characteristics can only evaluate the performance of OLEDs after they start to conduct current, the impedance versus voltage (Z-V) characteristic can be used to explore more electrical properties of devices even before they were turned on. It has been demonstrated that the accumulation charges at organic interfaces in OLED devices can be observed by Z-V characteristics measurement. We carried out Z-V experiments on devices with several combinations of hole transport layers (HTLs) and electron transport layers (ETLs). We found that the accumulation charges are caused by the extremely different mobility of hole and electron in HTL and ETL, respectively.

About Chih-I Wu:

Professor Chih-I Wu Associate Chairman (Professor)

Professor Chih-I Wu joined the faculty of Graduate Institute of Photonics and Optoelectronics and the Department of Electrical Engineering of National Taiwan University in 2004. His research is on optical-electronic devices and materials, organic light emitting materials, metal-semiconductor interfaces, and heterojunctions in electronic devices. Prior to joining NTU, he worked at Component Research Lab of Intel in the US from 2000 to 2004. His work at Intel was mainly on developing the advanced VLSI process technology, such as Cu and low k interconnects, metal gate materials, and atomic layer deposition process.

Professor Wu got his B.S. degree from National Taiwan University and M.S. degree from Northwestern University, both in Physics. He received his Ph.D. from Department of Electrical Engineering at Princeton University in 1999. At Princeton he worked on the electronic structures of optical-electronic semiconductors, including nitride-based semiconductors and organic thin films for light emitting diodes.

Professor Wu’s group is the only group to successfully develop the analytical solution to explain the current-voltage characteristics of OLED with presence of trap distribution in the organic materials. His lab is also the first group to propose the origins of turn-on voltage of OLEDs, which is determined by the interfacial energy level alignment in organic materials. He also designed and set up a home-made system which contains both OLED deposition apparatus and X-ray/UV photoemission spectroscopy with in-situ vacuum transfer mechanics. This unique system allows his lab to detect the electronic structures at the hetero-junction of organic devices and to understand the carrier transport behaviors in OLEDs.

Hosted by: Professor Larry Coldren